1. Field of the Invention
The present invention relates to an image-forming apparatus which is capable of forming a high-quality halftone image. More specifically, the invention relates to an electrophotographic image-forming apparatus having a charging device which charges a photosensitive member by using a oscillatory voltage source.
2. Description of the Related Art
Recently, image-forming apparatuses, e.g., plain paper copying machines, having a function which forms a high-quality halftone image corresponding to photographs and gravure, close to an image formed by an off-set printing machine, are requested. When the image formed by this type image-forming apparatus includes a low density area and a medium density area which is a halftone area, the image is not very good. Therefore, it is important to improve the reproducibility of the image for these density areas.
As one method which responds to this request, a screen method is disclosed by TRIKEPPS CO. LTD. "COLOR HARD COPY PRINT TECHNOLOGY," published Dec. 22, 1987. The screen method has a characteristic in an exposure process. An image-forming apparatus using the screen method has a halftone screen inserted between a photosensitive member and an original. This image-forming apparatus forms an electrostatic latent image which is decomposed into many dots, on the photosensitive member. As a result, the reproducibility of the image for the medium density area is improved.
However, the screen method has problems which are a reduction of the maximum density, a reduction of the resolution, and a reduction of the reproducibility of a character image.